Filter units having filter media are used to filter particles and others from gas streams. In the applications that require high collection efficiency, various filter media, for example, filter media including porous fluororesin membranes such as porous polytetrafluoroethylene (PTFE) membranes, filter media (glass filter media) obtained by adding binders to glass fibers and forming the resulting mixtures into paper sheets, and filter media (electret filter media) obtained by forming meltblown nonwoven fabrics into electrets, are used.
In particular, filter media including porous fluororesin membranes (porous fluororesin membrane type filter media) have the advantages of being less susceptible to problems such as formation of fibrils and self-dusting and of being less likely to cause an increase in pressure drop while in use. The porous fluororesin membrane type filter media also have the properties of fluororesins themselves. For example, since they have low friction coefficients and are very slippery, it is easy to remove collected dust by applying an impact to the porous membranes. Because of these many advantageous properties, the filter media including porous fluororesin membranes (in particular, porous PTFE membranes) are expected to be increasingly used in the future.
Porous fluororesin membranes are generally very flexible materials. On the other hand, filter media for filter units through which a large volume of air passes, such as, in particular, filter units for use in vacuum cleaners, are required to have a certain degree of rigidity so that they are resistant to being significantly deformed by the large volume of air. In response to this requirement, filter media including porous fluororesin membranes have a configuration in which an air-permeable support member as a reinforcing member is laminated on a porous fluororesin membrane as an air-permeable member. Various materials such as woven fabrics, nonwoven fabrics, metal or resin meshes, metal or resin nets, and resin foams are used for air-permeable support members. Nonwoven fabrics are often used because they are inexpensive. Nonwoven fabrics used for this purpose are generally those containing low melting point thermoplastic resins such as polyethylene and thus having thermally adhesive properties, like core-sheath fibers having a polyethylene terephthalate core and a polyethylene sheath. Such a nonwoven fabric is bonded to a porous fluororesin membrane by heat lamination to form a laminate thereof (see, for example, Patent Literature 1).